In electric vehicles and hybrid vehicles, it is a common practice to use the electric motor which is connected to the drive axle of the vehicle as a generator that produces electric power and provides a braking force at the same time when decelerating the vehicle. This is called as regenerative braking. However, the regenerative braking is generally inadequate for providing all of the need for the braking force, and it is common to combine the more conventional hydraulic (friction) braking with the regenerative braking. JP 2008-143256A discloses an electronically controlled brake-by-wire system that controls the hydraulic brake and the regenerative brake in a coordinated manner.
In a brake-by-wire system, a target brake force is set by an input amount given by a depression stroke of the brake pedal (pedal stroke), and the target braking force is distributed between the hydraulic braking and the regenerative braking. The hydraulic braking is based on the use of a motor actuated cylinder which is actuated by an electric motor, and supplies a brake fluid pressure for operating the wheel cylinders.
The ABS (anti-lock brake system) for preventing the locking of wheels at the time of braking is widely used in the existing vehicles, and performs the functions thereof by controlling the brake fluid pressure that is supplied to the wheel cylinders. JP 2007-331538A discloses an ABS system, and such an ABS system can be combined with a brake-by-wire system as an extraneous system that involves an extraneous brake fluid control operation. The VSA (vehicle stability assist) system is another example of systems that are extraneous to the main vehicle brake system but performs the functions thereof by acting upon the brake fluid pressure that is distributed to different wheels.
In the brake-by-wire system, the motor actuated cylinder is operated according to the target brake force or the target brake fluid pressure which is allocated to the hydraulic brake. The control of the brake fluid pressure is typically based either on the cylinder stroke of the motor actuated cylinder or the motor current of the electric motor of the motor actuated cylinder (or the force applied to the piston of the motor actuated cylinder).
When the control process is based on the cylinder stroke, the target cylinder stroke of the motor actuated cylinder is determined according to the target brake fluid pressure by taking into account the loss property of the brake fluid path between the motor actuated cylinder and the wheel cylinder, the cylinder stroke is converted into rotational angle of the electric motor, and the electric motor is operated by a feedback control so as to cause the cylinder stroke to agree with the target cylinder stroke.
When the control process is based on the motor current, the target motor torque is determined according to the target brake fluid pressure by taking into account the specifications or configurations of the motor actuated cylinder and the reduction gear unit of the electric motor, the motor torque is converted into motor current, and the electric motor is operated by a feedback control so as to cause the motor torque to agree with the target motor torque.
When the control process is based on the cylinder stroke, the amount of the brake fluid that is required for producing the target brake fluid pressure is used as the target value. As this target value or the amount of the brake fluid gives the direct measure of the braking force, a high responsiveness can be achieved both in normal braking and in combined braking (combining both the regenerative braking and the hydraulic braking), and the braking force can be produced at a relatively high precision. However, as the computation of the target value based on the cylinder stroke is made on the basis of the fluid loss property, if any fluctuation in the fluid pressure occurs, the actual fluid pressure may excessively overshoot the target fluid pressure, and this may cause the deviation between the target fluid pressure and the actual fluid pressure to persist more than desired. Such fluctuations of fluid pressure may be caused by an extraneous brake fluid control operation performed by an extraneous system based on the use of the brake system such as a ABS, a traction control and other VSA systems.
When the control process is based on the motor current, the motor torque that is required to produce the target fluid pressure is used as the target value so that the actual fluid pressure may be made to agree with the target fluid pressure without relying on an accurate estimation of the fluid loss property. However, the amount of the brake fluid that is required for producing the target fluid pressure is not considered. Therefore, as compared to the case where the control process is based on the cylinder stroke, the responsiveness is lower, and the coordinated control of the normal braking and the regenerative braking may be performed only with a reduced responsiveness.